settlement_of_shallow_foundations_on_granular_soils (Lutenegger ang DeGroot)

s = settlement (in inches)
q = net footing stress (in tsf)
N =uncorrected (field) blowcounts
B =footing width (in ft.)
Cw = water correction
= 2- (W/2B) ~ 2.0 for surface footings
= 2- 0.5 (D/B) ~ 2.0 for fully submerged,
embedded footing; W ~D
Cd =embedment correction
= 1 - 0.25 (D/B)
[5.5]
[5.6]
[5.7]
where:
W = depth of water table (in ft.)
D = footing depth (in ft.)
The uncorrected SPT blowcount data are used in calculating settlement, however, if the sand is
dense, saturated and very fine or silty (e.g., abundant fines content), it was recommended that the
blowcount should be corrected according to:
N,= 15 +0.5 (N -15)forN> 15 [5.8]
The correction for water table applies to cases where ground water is at or above the base
of the footing (complete submerged case). For partial submergence (water located between D and
D+B) a correction factor is given for surface footings (no embedment) only. In common practice,
the water correction is often omitted from the settlement estimates using this method since the
method is generally considered to be overly conservative.
5.2.2 Meyerhof (1956, 1965)
Meyerhof (1956) suggested that the allowable bearing pressures for a footing on granular
soils could be estimated based on the results of SPT blowcounts. The allowable pressure includes
a minimum factor of safety of 3 against bearing capacity failure and may be less than the safe
bearing pressure ( q, 1 /3) if the settlement resulting from the safe bearing pressure is excessive.
Assuming that the allowable bearing pressure cause 25 mm (1 in.) of total settlement, Meyerhof
(1956) proposed the following expression for dry and moist sands:
q, = N/8
q, = N(1 + 1/B) 2 /12
q, = N/10
where:
(forB ,; 4ft.)
(forB> 4ft.)
(approximately, for any B)
[5.9]
[5.10]
[5.11]
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